Security alarm systems that detect and actuate an alarm when a door, window, or movable closure member of another entryway is opened conventionally employ a magnetic switch assembly such as illustrated in FIG. 1. Prior art switch assembly 10 includes an elongate magnet 12 and a corresponding magnetic switch 14, which are recess mounted in movable closure member 16 and fixed frame member 18, respectively, so that magnet 12 and switch 14 are in the juxtaposed axial alignment shown when the door, window, or other entryway is in a closed position. FIG. 1 shows that the housings for elongate magnet 12 and magnetic switch 14 are of the same size.
FIG. 2 illustrates the magnetic and electrical components in a schematic depiction of prior art security switch assembly 10. When magnet 12 and switch 14 are placed in axial alignment within a predetermined gap 20, magnetic field 22 and electrical contacts 24 interact so as to place switch 14 in a nonalarm state. Beyond gap 20 is a break distance 26 delineating the threshold proximity between magnet 12 and electrical contacts 24 at which the nonalarm state of switch 14 is maintained. Gap 20 and break distance 26 are measured between the opposed end faces of the housings for magnet 12 and magnetic switch 14. Between gap 20 and break distance 26 is a zone in which magnetic flux is of insufficient density to permit interactability with electrical contacts 24. Opening closure member 16 so as to move magnet 12 beyond break distance 26 places electrical contacts 24 outside of the interactive zone of magnetic field 22. Switch 14 thereby assumes an alarm actuation state.
Acceptable gap and break distances between the magnet and magnetic switch components of security switch assemblies have been established by industry standards based on customary mounting specifications, safety considerations, and market acceptance. Such acceptable gap distances are 12.5 millimeters (0.5 inch) for standard gap mounts and 25.5 millimeters (1.0 inch) for wide gap mounts.
Failure to comply with such well-established gap and break widths in mounting security switch assemblies gives rise to numerous problems, including serious safety hazards. An overly narrow gap fails to provide acceptable tolerances for accommodating standard clearances and expected irregularities, which result in misalignments and spaces between frames and corresponding closure members. If the gap between the switch and magnet components of an installed switch assembly has an irregularly wide space below standard tolerances, an increased false alarm actuation rate may result. A gap in excess of standard widths, on the other hand, introduces increased safety risks. This results from the risk that a closure member could be moved slightly ajar without actuation of the alarm. Thus, a potential burglar might be able to crack a door open far enough to tamper with and deactivate the alarm while the magnet remains within the threshold break distance. A very wide gap could even permit entry or unlocking of an inside latch or alternative entryway. It can thus be seen that compliance with established gap widths is important.
Referring again to FIG. 2, in order for a magnet to emanate a magnetic field 22 of sufficient strength to interact with electrical contacts 24 within acceptable tolerances for gap 20 and break distance 26, a certain magnetic flux density of magnet 12 must be sustained. Sustenance of the flux of a magnetic material is defined by its intrinsic coercive force, which is defined by its resistance to demagnetization forces. The intrinsic coercive force of a material is measured in oersteds.
Conventional security switch magnetic materials, such as alnico (aluminum, nickel, cobalt) are characterized by low levels of intrinsic coercive force. The low resistance to demagnetizing forces of alnico magnets results in sizable loss of magnetic flux density relative to a slight decrease in magnetic force. As a result, an alnico magnet cannot recover its original flux output without being remagnetized. The low level of magnetic force places geometric constraints on a conventional security switch magnet in which relatively very high length-to-diameter ratios are required to provide sufficient intrinsic coercive force to maintain acceptable magnetic flux levels for the expected life of a security switch assembly. A typical prior art alnico security switch magnet that is not susceptible to demagnetization has a length-to-diameter ratio of greater than 4-to-1 or more; therefore, an alnico magnet having a length-to-diameter ratio of less than 4-to-1 is susceptible to demagnetization. Due to such length-to-diameter constraints, conventional security switch magnets are bulky and elongate in configuration, as shown in FIGS. 1 and 2.
Such length-to-diameter constraints have rendered prior art security switch magnetic switches obtrusive and their installation invasive. Boring deep recesses to mount these elongate cylindrical magnets entails awkward and time-consuming procedures. The problems with installation are magnified when a security switch assembly is mounted in a tight space. Deep recess preparation in such a tight space is inconvenient, entailing the manipulation of tools around corners and proximate surfaces. This can result in imprecise recess boring and resulting misaligned installment. A most unfortunate consequence could be an unacceptable gap or break distance giving rise to potential safety hazards, as well as technical problems.
Another problem with prior art magnetic security switches results from boring recesses of a depth commensurate to the elongate cylindrical configuration of prior art magnets. Recess mounting of prior art magnets generally damages surface materials, such as laminates and veneers. For example, for vinyl clad windows, deep recesses damage the subsurface structural integrity located more than 4.0 millimeters (0.16 inch) below the surface of the component on which the security switch is mounted. Doors and window frames in which security switch magnets are mounted typically measure less than 3.8 to 6.35 centimeters (1.5 to 2.5 inches) in width. At this thickness, wood material, from which such structures are most often composed, becomes more prone to splintering and splitting when subject to deep boring.
A particular disadvantage of recess boring required to install elongate prior art magnets results from the frequent, if not inevitable, puncture of or damage to vinyl cladding or other thermal insulation material covering a movable closure structure, which is typically a window, on which such magnets are installed. Thus, damaging the insulation violates the integrity of the thermal field and reduces thermal efficiency of the relevant interior. As a result, acceptable thermal ratings, which are required by some building codes, are not attained. Perhaps most significantly, warranties on such insulation materials are typically invalidated because of punctured or damaged insulation.
The numerous drawbacks associated with prior art security switch magnets are compounded when security switch assemblies are installed in tight-fitting structures with close clearances, particularly windows. For instance, puncturing of vinyl cladding when mounting a security switch in windows having tight thermal fits causes particular perplexity, for obtaining an improved thermal rating and/or insulation warranty is often a primary purpose for installing such windows.
Until now, there has been no magnetic security switch assembly addressing the above problems with the prior art. There persists, therefore, an ongoing need for a readily and noninvasively installable security switch assembly having an unobtrusive magnet with acceptable magnetic force and density.